Diamines and polyamines of the diphenylmethane series (MDA) are understood to be amines and mixtures of amines of the following formula (I):
wherein n represents a natural number ≥2.
For compounds and mixtures of compounds with n=2, the term “monomeric MDA” (hereinafter MMDA) is also conventionally used, whilst compounds and mixtures of compounds with n>2 are conventionally referred to as “polymeric MDA” (PMDA). For the sake of simplicity, mixtures containing compounds with n=2 and n>2 side by side are hereinafter referred to as MDA (diamines and poly amines of the diphenylmethane series).
The most important isomers of MMDA are 4,4′-MDA, 2,4′-MDA and 2,2′-MDA:

4,4′-MDA is sometimes called the para-isomer, whereas both 2,4′-MDA and 2,2′-MDA individually or grouped together are sometimes referred to as ortho-isomers.
As a synonym for the PMDA, the terms “higher homologues of MDA” and “oligomers of MDA” can be found in the literature.
MDA is an extremely suitable starting material from which—optionally after further purification—the respective di- and polyisocyanates (hereinafter MDI) that represent an important raw product for polyurethane systems, for example, can be obtained by phosgenation. At the same time, the aliphatic systems that are obtained from MDA by hydrogenation of the aromatic ring also play an important role as paint resins.
Of the many conceivable methods described in the literature for the production of MDA, manufacture from the aniline-formaldehyde condensation product (known as aminal) is the most important because it is the most economically advantageous. This process can be illustrated in idealised form by means of the following diagram:

Depending on the variant, the condensation product (the “aminal”) is produced first and then rearranged in the presence of a catalyst; alternatively, the condensation itself is performed in the presence of a catalyst under rearrangement conditions.
The rearrangement is catalysed by acids. Usually, solutions of strong acids, such as hydrochloric acid, sulfuric acid, and/or phosphoric acid, are employed in either variant, giving rise to the formation of an amine salt, which generally is subsequently neutralized with a base. For this purpose, strong bases such as sodium hydroxide are frequently used. This process suffers from several disadvantages:
Obviously, large quantities of strong acid are required, which is undesirable from an economic as well as an ecological perspective. In addition, use of strong acids may necessitate use of corrosion resistant materials in the equipment. Such construction materials are often expensive. Furthermore, neutralization of the strong acids employed with bases inevitably leads to the formation of large quantities of salts, which must be disposed of safely. These salts may also be contaminated with organic products, which need to be discharged, resulting in increased production costs. Additionally, substantial quantities of waste water are generated by this process, requiring additional processing capacity for the further treatment of the waste water before this can be safely discharged into a sewerage system.
A whole series of suggestions for the industrial implementation of the rearrangement has therefore already been made in order to overcome these disadvantages. Generally, the approach is taken to substitute mineral acids such as hydrochloric acid by solid acids, thereby simplifying the separation of MDA and catalyst as well as, at least in principle, allowing to reuse the catalyst.
However, a feasible method for the production of MDA avoiding mineral acids must meet the following conditions, for example:    a) Quantitative yields: an intermediate-free (aminobenzylaniline-free) product must be obtained in order to ensure that it is capable of being phosgenated (these can be extremely troublesome in the subsequent processing of the MDA to MDI (phosgenation).    b) Isomer distribution: similarly to the mineral acid-catalysed method, the product composition must be able to be controlled to some extent by varying the process parameters.    c) Service life: a catalyst used in industry must achieve an economic service life with high space-time yields before its activity can be restored by means of regeneration.    d) Foreign substances: the catalyst used must release no trace components in the product that have a negative influence on product quality. In addition, the method must cause no foreign matter, e.g. in the form of a solvent that is foreign to the system, to be brought into the reaction mixture.
Clays such as attapulgite and kaolin are stable up to 180° C. and can be regenerated through calcination (cf. U.S. Pat. Nos. 4,039,580, 4,092,343 and 4,294,987). However, in the production of MDA they exhibit a low selectivity to the 4,4′-isomer (the ratio of 4,4′-MMDA to 2,4′-MMDA being approximately of from 2 to 4), as a result of their weak acidity. The intolerance to water in the feed (max. 0.15 wt.-%) is a further drawback in industrial application, since a distillation of the intermediate aminal to reduce the water content is cost-prohibitive.
In contrast, amorphous silica-alumina (ASA) materials provide higher activities, an increased water tolerance (up to 3 wt.-%), and stronger acid sites, leading to improved selectivity, the ratio of 4,4′-MMDA to 2,4′-MMDA being approximately 5 (cf. U.S. Pat. Nos. 3,362,979, 3,971,829 and BE1013456A6). However, an even higher ratio of para to ortho isomers would be desirable.
It is known from U.S. Pat. No. 3,860,637 that rearrangement of the aminal using amorphous silicon-aluminium-mixed oxide cracking catalysts results in high yields of 4,4′-isomers when the reaction is performed in the presence of added ortho-isomers. These preferentially react to higher oligomers of MDA. A high proportion of PMDA is therefore conventionally obtained, which has to be separated from the desired 4,4′-isomer. This process requires the additional step of recycling the ortho-isomers initially formed.
A general problem of catalysts in MDA synthesis is deactivation due to inefficient removal of reaction products (Alberto de Angelis et al., Ind. Eng. Chem. Res., 2004, 43, 1169-1178). Accordingly, highly mesoporous silica-alumina samples such as MCM-41 have been tested (Carlo Perego et al., Appl. Catal., A, 2006, 307, 128-136). However, selectivity was not sufficiently high, and the synthesis of the catalysts is costly.
WO 2010/019844 discloses the application of solid acid silica-metal oxide catalysts in the synthesis of MDA. The conversion is below 100% with ABA concentration of >1%.
Zeolites provide strong Brønsted-acid sites resulting from tetrahedrally coordinated aluminium incorporated in the crystalline framework. Additionally, zeolites can provide shape selectivity. Compared to other zeolites, Faujasite (FAU) and beta (BEA) zeolites have been described as most active catalysts in MDA synthesis (Tsuyoshi Kugita, et al., Catal. Today, 2006, 111, 275-279. Zeolite beta was identified as the most active zeolite, but provided an undesirably low para/ortho ratio of about 2.5, which was attributed to shape-selective properties of the zeolite micropores (Avelino Corma et al., Chem. Commun., 2004, 2008-2010).
EP 1 055 663 B1 discloses the use of zeolitic materials with a spaciousness index between 2.5 and 19 for catalysing aminal rearrangement with high conversion above 95% to MDA. BEA is mentioned as preferred framework.
Silanized solid materials having a spaciousness index between 2.5 and 19 are disclosed in EP 1 355 874 B1 as catalysts for MDA synthesis. The ratios of 4,4′- to 2,4′-MDA described therein are, depending on the catalyst, between 1.15 and 3.7. Silanization is rather ecologically unfriendly since the utilized precursor tetraorthosilicate have to be produced from pure silicon over silicon tetrachloride as intermediate (Inorganic Silicon Compounds, W. Simmler in Ullmann's Encyclopedia of Industrial Chemistry, 2000, DOI: 10.1002/14356007.a24_001).
A similar approach is followed in the teaching of EP 1 381 589 B1. Zeolite materials are post-treated with phosphoric as well as boric acid. The catalysts thus obtained reveal a certain activity in MDA synthesis. The ratio of 4,4′-MDA to 2,4′-MDA was increased from 2.2 to a maximum of 6.95.
EP-A-0 264 744 describes the condensation of aniline with trioxane or free formaldehyde and the rearrangement to MDA using solid boron, titanium and iron-containing zeolites. Simultaneous condensation and rearrangement as well as isolation of aminobenzylanilines with subsequent rearrangement to MDA are both disclosed. Although high monomer selectivity was obtained by rearrangement of the intermediate aminobenzyl anilines to MDA (approx. 90 mol % MMDA in the product after removal of aniline), complete conversion is not achieved. Furthermore the reaction is preferably performed in an additional solvent which is undesirable from an economic perspective.
WO/0158847 A1 describes a process for the production of MDA containing high amounts of MMDA with low ortho-content via a solid acid-catalysed rearrangement of a condensation product from aniline and formaldehyde or another methylene group-supplying agent like trioxane or para-formaldehyde. Preferred solid catalysts are FAU zeolites. The invention is directed at a process which produces an MDA with as little PMDA as possible. However, whilst very high monomer contents might be desirable for certain special applications, it is not desired as a rule in industry to avoid the formation of PMDA since the latter has proven to be useful in many applications. In addition, the process requires the use of highly pure aniline with a very low content of aliphatic amines.
Zeolitic materials based on faujasite structure were utilized as catalysts in MDA synthesis according to method described in CZ 0 301 977 B6. High yields of monomeric MDA were obtained, whereas an A/F of 10 was applied. The recipe makes the industrial process economically unattractive since excess aniline has to be removed from the MDA if it is later on utilized as precursor for MDI.
Exchange of the protic sites in zeolites by alkaline metals is described in JP 2012 131720 A. Lithium-modified faujasite-type zeolites show activity in the rearrangement towards MDA. However, the conversion of the intermediate aminobenzylanilines to MDA is incomplete.
The solid acid catalysed MDA synthesis is reported in JP 2012 250971 A. Silica-alumina and Y zeolites are described as solid catalysts. However, the conversion of the intermediate aminobenzylanilines to MDA is incomplete.
EP-A-0 329 367 describes the rearrangement of a dried aminal over metal-containing zeolitic catalysts for the purpose of selectively producing MMDA. The aminal is rearranged isothermally at 120° C. using steam-dealuminated HY zeolites and fluorinated derivatives thereof to give a MDA, which although consisting of approx. 94 mol % (relative to aniline-free solution) of MMDA is characterized by incomplete conversion of the intermediates to the MDA. Approximately 5 mol % of PMDA are additionally formed.
Concerning steam-dealuminated HY zeolites, mechanistic studies for aminal rearrangement can be found in Michael Salzinger et al., Green Chem., 2011, 13, 149-155. Based on kinetic time profiles, it is concluded in this reference that the reaction order of the first step (rearrangement of the aminal to yield aminobenzylanilines) is one, whereas that of the second step (rearrangement of aminobenzylanilines to MDA) is two. Moreover, this study reveals either film or pore diffusion limitations especially for the first step, leading to decreased turnover at the active sites and negatively affecting the space-time-yield of the overall process.
Through delamination of layered, template-containing zeolite precursors through swelling agents and ultrasound, the acid sites can be made accessible to bulkier molecules, and diffusion limitations avoided (Pablo Botella et al., Appl. Catal., A, 2011, 398, 143-149, WO 03/082803 A1). The catalytic activity of these zeolites with respect to aminal conversion employing a molar ratio of aniline to formaldehyde (hereinafter “A/F”) of 3 is described. The MDA thus obtained contains approximately 25% of PMDA. The exfoliation process results in reduced acid strength compared to zeolitic materials (cf. Avelino Corma et al., Microporous Mesoporous Mater., 2000, 38, 301-309) and thus in a low 4,4′-MDA/2,4′-MDA ratio. Furthermore, this approach is limited to layered zeolites, whose synthesis relies on the application of sacrificial templates and surfactants. The relatively high cost of the zeolites, combined with the excessive consumption of surfactants in the delamination process, render an industrialization of this approach unattractive.
Desilication by alkaline treatment enables to generate mesopores in zeolites while increasing the aluminium content (Javier Pérez-Ramírez et al., Chem. Soc. Rev., 2008, 37, 2530-2542). Mesoporous zeolites thus produced have been tested in various catalytic reactions (liquid-phase degradation of high density polyethylene, cumene cracking, oligomerisation of styrene, liquid-phase benzene alkylation, butane aromatisation, conversion of methanol to gasoline and methanol to propylene, ion-exchange and redox catalysis).
Martin Spangsberg et al., Catal. Today, 2011, 168, 3-16 discloses examples of the successful utilization of hierarchical zeolites, i.e. zeolites which have been subjected to a defined post-synthetic design process so as to result in zeolitic structures featuring at least one additional level of porosity besides the intrinsic micropore system characteristic of zeolites, in various acid- and base-catalysed reactions of scientific and industrial relevance. The reference mentions alkylation reactions, methanol to hydrocarbon and aromatization reactions, isomerization reactions of hydrocarbons, cracking reactions, condensation reactions such as aldol condensations, esterification reactions, acetalisation reactions, and other reactions such as the Beckmann rearrangement, epoxidation and hydroxylation reactions, hydrotreating reactions and the decomposition of nitrogen oxides. No reference is made to MDA synthesis.
Danny Verboekend et al., Catal. Sci. Technol., 2011, 1, 879-890 and Danny Verboekend et al., Chem. Mater., 2013, 25, 1947-1959 describe various post-synthetic strategies to obtain hierarchical zeolites. These strategies have evolved far beyond the point of conventional desilication. The catalytic and adsorptive properties of the hierarchical zeolites are evaluated in the Chem. Mater. reference, disclosing their use in the alkylation of toluene with benzyl alcohol or isopropyl alcohol and the Knoevenagel condensation of benzaldehyde with malonitrile. The use of hierarchical clinoptilotite in alkylations is envisaged. The Catalysis Science & Technology reference discloses the use of hierarchical zeolites in a wide range of reactions mentioning isomerisation, alkylation, acylation, aromatisation, cracking, pyrolysis, methanol-to-hydrocarbons conversions, gas-oil hydrocracking, pyrolysis of low-density polypropylene and biocatalysis. MDA synthesis is not mentioned in any reference.
The use of a BEA (=beta) zeolite which was alkaline-treated in the absence of a pore-directing agent in MDA synthesis is disclosed in Michael Salzinger et al., Appl. Catal., A, 2011, 393, 189-194. The reference explicitly discourages the skilled person to use alkaline treatment, be it in the presence or absence of a pore-directing agent, for zeolites other than BEA zeolites (see page 193, right column, second paragraph) due to the destruction of the zeolite framework. The parent zeolite is first treated with NaOH and then with NH4Cl (so as to replace sodium by ammonium). Even for the beta zeolite, the authors report show that already after 15 min of alkaline leaching, the BEA framework is severely damaged based on XRD diffractograms.
A regeneration process of beta zeolite having been employed in MDA production is described in EP 1 294 481 B1 by interrupting the feed of aminal and feeding aniline (as regenerating agent) instead. In addition, the initial temperature was raised from 180° C. to 250° C. Before and after regeneration, the catalysts exhibited the same characteristics with regard to isomer as well as polymer composition.
Attempts have also been made to perform the rearrangement of aminal via aminobenzyl aniline to MDA in several steps, for example in two steps, using solid acids in more than one step. U.S. Pat. No. 4,039,581 describes the rearrangement of the aminal using solid acids, whereby the aminal is first dried and then rearranged using zeolites, for example, in several reaction stages with increasing temperature. A temperature of 100° C. is not exceeded. It is assumed that high temperatures in the presence of water would be damaging to selectivity. A full rearrangement of the aminobenzyl aniline intermediates to the MDA cannot be achieved under these conditions. MDA with an MMDA content of approx. 90 mol % in the aniline-free mixture is obtained as product.
WO 2010/072504 A1 describes a continuous process for the synthesis of MDA using solid catalysts. Besides others, different types of silica-alumina as well as clays serve in the first stage to convert aminal towards intermediates like aminobenzylanilines. In the second stage, MMDA and higher homologues are obtained by treating the intermediate mixture with solid catalysts like zeolites, delaminated zeolites or ordered mesoporous materials. The yield of monomeric MDA decreased in general by increasing run-time depending on the choice of catalyst. The necessity of using two different types of catalysts renders the process undesirably complicated.
To summarise, there has been considerable progress in the area of solid zeolite catalysis for MDA synthesis. However, up to now no process is known which really has the potential to replace traditional HCl catalysis on a large industrial scale. Therefore, there still exists a need to improve solid zeolite catalysis in MDA synthesis. The present invention addresses this need.